Tires are sometimes desired with treads for promoting traction on wet surfaces. Various rubber compositions may be proposed for tire treads.
For example, tire tread rubber compositions which contain high molecular weight, high Tg (high glass transition temperature) diene based elastomer(s) might be desired for such purpose particularly for wet traction (traction of tire treads on wet road surfaces). Such tire tread may be desired where its reinforcing filler is primarily precipitated silica which may therefore be considered as being precipitated silica rich.
Such elastomers typically have a high uncured rubber viscosity (e.g. Mooney, ML1+4, viscosity) and thereby often contain a petroleum based rubber processing oil to reduce the rubber composition's uncured viscosity and to thereby promote more desirable processing conditions for the uncured rubber composition. The petroleum based rubber processing oil can be added to the elastomer prior to its addition to an internal rubber mixer (e.g. Banbury rubber mixer) or be added to the rubber mixer as a separate addition to reduce the viscosity of the rubber composition both in the internal rubber mixer and for subsequent rubber processing such as in a rubber extruder.
Here, the challenge is to reduce the cured stiffness of such tread rubber compositions, as indicated by having a lower storage modulus G′ at −20° C., when the tread is intended to be used for low temperature winter conditions, particularly for vehicular snow driving.
It is considered that significant challenges are presented for providing such tire tread rubber compositions for maintaining both their wet traction while promoting low temperature (e.g. winter) performance.
To achieve such balance of tread rubber performances it is proposed to provide a tread rubber composition containing only low Tg rubbers, such as cis 1,4-polybutadiene rubber, styrene/butadiene rubber and optionally cis 1,4 polyisoprene rubber having relatively low Tg values below −55° C. to improve, or beneficially lower, the stiffness of the cured rubber composition at −20° C. to improve winter performance in contrast to using a higher Tg SBR which would be expected to significantly increase the cured stiffness of the rubber at lower winter temperatures. To meet such challenge of providing good winter performance while maintaining wet traction for the tire tread it is also desired to use a silica-rich filler reinforcement for the tread rubber composition containing the low Tg elastomer(s) to promote wet traction combined with promoting a reduction in its cured stiffness at low temperatures, and replacing the petroleum based rubber processing oil (e.g. comprised of at least one of naphthenic and paraffinic oils) with a vegetable triglyceride oil such as, for example, soybean oil to reduce its uncured rubber processing viscosity and to further reduce the Tg of the rubber composition itself to thereby promote a lower cured stiffness of the tread rubber composition at lower temperatures which will thereby positively impact and beneficially promote the low temperature winter performance of such rubber compositions. The innovation of this approach relies on the use of high Tg (comparatively higher Tg than the SBR and polybutadiene rubbers) liquid diene-based polymer (particularly a low viscosity, high Tg, styrene/butadiene polymer) and traction promoting resin in the tread rubber composition, particularly at a relatively high resin loading, to promote wet traction of the sulfur cured tread rubber which contains the vegetable triglyceride oil and only low Tg solid, particularly higher molecular weight, elastomers.
Exemplary of past soybean oil usage, and not intended to be limiting, are U.S. Pat. Nos. 7,919,553, 8,100,157, 8,022,136 and 8,044,118.
However, while vegetable oils such as soybean oil have previously been mentioned for use in various rubber compositions, including rubber compositions for tire components, use of soybean oil together with precipitated silica reinforced combination of low Tg diene based elastomer(s), liquid diene based polymers and traction resin(s) for tire treads to aid in promoting a combination of both wet traction and winter tread performance is considered to be a significant departure from past practice.
In the description of this invention, the terms “compounded” rubber compositions and “compounds” are used to refer to rubber compositions which have been compounded, or blended, with appropriate rubber compounding ingredients. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The amounts of materials are usually expressed in parts of material per 100 parts of rubber by weight (phr).
The glass transition temperature (Tg) of the solid elastomers and liquid polymer may be determined by DSC (differential scanning calorimetry) measurements, as would be understood and well known by one having skill in such art. The number average molecular weight (Mn) of the solid elastomers and liquid polymer may be determined by GPC (gel permeation chromatography) measurements as would be understood and well known by one having skill in such art. The softening point of a resin may be determined by ASTM E28 which might sometimes be referred to as a ring and ball softening point.